Bifocal lens



Jan. 9, 1923. H. FENNELL 1,441,565.

BIFOCAL LENS".

FILED FEB. 23. 1922.

Invenlor j Allomey:

Patented dan. 9, i923.

STATE PATE,

HARRY FENNELL, OF EVERETT, MASSACHUSETTS.

IBIFOCAL LENS.

i' Applicatiomled February 23, 1922. i Serial No. 538,502.

To all 'whom t may concern.:

Be it known that I, HARRY FENNELL, a citizen of the United States, residing at Everett, in the county of Middlesex and State of Massachusetts, have invented certain new and useful Improvements in Bifocal Lenses, of which the following is a full, clear, Vand exact description.

My invention is designed for the production lof an improved form of bifocal lens which will have all of the advantages of the well known Kryptok fused bifocal and will overcome certain objections thereto.

The Kryptok bifocal consists of a body portion of crown glass and a segment of flint glass fused in a reces in the body portion. Both sides of the finished lens have even or continuous surfaces, and one side of the readin segment is exposed. Flint glass is much so ter than crown glass-and contains lead in its composition. As a result, the exposed surface of the reading segment of a ,Kryptok lens is more easily scratched than the remaining portion of the lens and after long use becomes dull or partially opaque. In

addition, oxidization or other chemical action takes place on the exposed surface of the segment. Exposure to the atmosphere, after a while, causes the formation of a yellowish film on the outer surface of the reading segment, and perspiration causes the formation of awhite film thereon.

rPhe object of my invention is to provide a vbifocal lens of the general type of the Kryptok lens in which the scratching or abrading of the reading segment, as well as the formation of a film thereon from chemical action,

is e'ectually prevented.

A further object of the invention is to minimize or completely remove the chro matic colors present in the Kryptok lens in Vthe high reading additions, due to the fact that the dispersion of the flint glass is not overcome by the crown glass used. y

Other objects of the invention will hereinafter appear.

The invention resides in the lens and in the blank from which it is made, and consists of a bifocal lens or blank comprising a body portion and reading segment composed of glass of different indices of refraction, the reading segment being enclosed within the body portion and integrally united thereto.

It also consists of a bifocal lens or blank comprising a body portion, a reading segment fused into a recess in the body portion and a layer of glass covering said segment and fused thereto and to the body portion, the layer of glass being of the same index of refraction as the glass o f the body portion but of a different index of refraction from that of the segment.

It also consists in certain features, characteristics, and details of construction which will be hereinafter more fully described and claimed.

In the drawings forming part of the specification Fig. 1 is a sectional view of a finished lens embodying my invention;

Fig. 2 is a face view of the same;

Figs. 3 and 4 are sectional and plan views respectively of the blank;

Figs. 5, 6, 7 and 8 are sectional views of the different elements from which the lens is made, indicating the successive steps in the manufacture of the lens, and

Fig. 9 is a sectional view of a blank showing in dotted lines how the same should be ground for the production of the lens.

My improved lens, as well as the blank therefor, is an integral structure comprising a body portion 1 and a reading segment 2, the latter being enclosed within and surrounded by the body portion. The .bodyv portion and reading segment are composed of glass of different indices of refraction, the former being preferably made of what is known as crown glass and the latter Yof what is known as flint glass. In order to obtain greater focal power through the reading field of the lens the segment 2 is made of flint glass which has a higher refractive index than the crown glass of the body portion 1. The opposite sides of the finished lens have even continuous surfaces, and both are of glass material of the saine index of refraction. The segment 2 irshown as seated in a spherical recess .in the` body portion 1 and as covered by .la layer or film of glass 3, ali. parts being integrally united.l

@ne Vmethod of producing my improved lens is as follows:

I first form a spherical recess ordepression 4 in the surface of a block 1 of crown glass, which constitutes the body portion of the lens or blank. This block may be'of circular, square or other suitable contour and is suiciently thick to accommodate the 11u y crown and flint glass.

recess 4 and provide the necessary material required to enable it to be ground away for the production of the proper curvatures on the surfaces of the finished lens. I then prepare what is known as a button, for inserting and fusing into the recess 4. This I do by fusing together two small circular discs of glass 5 and 6, whose diameter is approximately the same as that of the recess 4. The disc 5 is of glass having the same index of refractionas the block or body portion' 1, whereas the disc 6 is of glass having andifferent and preferably higher index of. refraction. That is, the discs 5 and 6 maybe respectively made of what are known as The adjoining surfaces of the two discs 5 and 6 are shown as plano, although this is not absolutely essential. After the discs 5 and 6 have been fused to each other, the flint glass side of the integrally united parts is ground and polished upon a` plus or convex curve, corresponding substantially with the curvature of the recess 4, the grinding being continued until a portion, at least, of the edge of the crown glass disc 5 is ground away, as shown in Fig. 7.V A button 7 is thus produced com-l posed of a lower portion 8 of flint glass and a relatively thin, fiat cover portion 9 of crown glass. In actual practice the thickness of the crown glass cover portion 9 need not be more than two or three millimeters` and the widest diameter of the same need not be more than two or three millimeters greater than the widest diameter of the flint glass portion. While the convex surface of the button 7 corresponds substantially with that of the recess4 in the body portion 1, I prefer that the same be ground upon a slightly stronger curve,that is, A upon a curve of shorter radius.

The button 7, is afterwards placed in the recess 4 in the block 1,-subjected to .heat .in a suitable furnace, and fused in the' usual manner well known inthe manufactureof Kryptok bifocals. The resultant product is a lens blank such as represented in Figs. 3

and 9 of the drawing,-that is, an integral o1; fused blank com osed cfa body portion 1 of crown glass an a readingsegment 2 of flint glass tected bythe body portion. As the different glass elements are fused together, an integral unitary -structure is produced in which the.

flint glass reading segment 2 is seated in the recess 4fof the crown glass body portion `1 and is covered and protected lby the crown glass layer 9; f

To produce a finished lens from this blahk, the opposite surfaces thereof are ground and polished and the lens cut toeye size and edged, ina manner well known inthe art. The grinding of the side of the blank 'containing the button 7 is permitted to proceed justl far enough to cut away a part ofthe wholly enclosed within and pro- -the flintVv glass reading segment is. enclosed within the crown glass body portion and that no portion of either surface thereof is exposed so that it may be scratched or subjected to chemical action from external substances.

In' order to overcome the objection of chromatism, present in the Kryptok bifocal and due to thel use of the crown and vflint glass elements which have different dispersive powers, the oportion 9 of the button 7 from which the layer or film 3 is'produced, may be made of glass having a different dispersive power from that of the body portion 1. It must be of glass of the same index of refraction as that of the body portion and to secure the proper result its dispersive power should be less thany that of the body portion. The lens thus made becomes in a measure achromatic. For example, what is known as hard crown glass having a refractive index of 1.512 and a dispersive index of 60.3 c

may be used for the body portion 1, and what is known as zinc crown glass having a refractive index of 1.512 and-a dispersive index of 57.8 may be used for the portion 9 of the button 7. y' A 1. A bifocal lens jor blank comprising a vbody portion and-.a reading segment composed of glass of different indices of refraction, .the e reading `segment ybeing enclosedl within the vbody portion and integrally united thereto. 3 e

2. A bifocal lens or blank having a crown glass. body p'ortion and a flint ,glass reading segment, the latter being enclosed within andA protected' by the body portion, and the two 'parts of the lens-being integral..

3. A bifocal lens or blank comprising a body portion, a recess in the body portion and a layer of glasfs covering said segment and fused thereto and to the 'body portion. the said layer of glass being of the same index of refraca. reading segment fused into tion as the glass of the body portion and crown glass body portion, a flint-glass reading segment fused into a recess in the body',

portion and la layer of crown glass covering said .segment and fused thereto and to the body` portion.

5; A bifocal lens or blank comprising a. to and to the body portion, the said layer of body portion of one index of'refraction and glass beingof the same index of refraction a reading segment enclosed thereby of a difas the glass of the body portion but of a ferent index of refraction, the glass of the different dispersive power, and of a different 5 body portion having dil'erent dispersive index'of refraction from that of the segl5 powers on opposite sides of the segment. ment.

6. A bifocal lens or blank comprising-a ln Witness whereof, I hereunto subscribe y body portion, a reading segment fused into my signature. a recess in vthe body portion and a layer of l0 glass covering said segment and fused there- HARRY FEN N ELL 

